In the art of well completions, subterranean formations holding hydrocarbon values such as oil and/or natural gas can comprise unconsolidated fine particulate materials. When a well penetrating such a formation is placed on production in order to recover the hydrocarbons therefrom, the fine particulate matter in the formation is carried into the well bore and can eventually block the production of hydrocarbons with silt.
In the production of hydrocarbons from such unconsolidated formations, it has been common to place a so-called gravel pack in the near well bore area. The gravel pack comprises a sized particulate, typically sand, which forms a filter bed which traps formation particulates in the interstices between the sand particles while allowing the flow of fluid hydrocarbon components to proceed to the well bore. In this manner, production of the hydrocarbon fluids can proceed for an extended period of time without a buildup of silt within the well which would otherwise inhibit or completely terminate production.
A gravel pack is typically placed in the near well bore area utilizing a well packer to isolate the desired formation and pumping the sand in a viscosified fluid through a screen or slotted liner to place the gravel pack sand in and adjacent to the well bore at the unconsolidated formation face. The placement fluid typically is viscosified by any of several natural and/or synthetic polymeric materials such as cellulosics, galactomannan gums or various synthetic polymers such as polyacrylamide. In high permeability formations however, viscous fluids alone can be inadequate to control fluid loss into the highly permeable surrounding zone. Particulate supplements are often necessary to decrease such fluid loss to the formation. Conventional particulate based fluid loss control systems are only moderately successful due to unknown downhole parameters such as permeability, pore throat size and particle size of the fluid loss control material. Additionally, clean up and removal of these materials following gravel pack placement is often difficult.
Particulate fluid loss control systems have the ability to effectively control fluid loss by forming a low permeability filter cake. While controlling fluid loss, the filter cake provides damage protection to the formation trapping incoming polymer residues and foreign particles residing in the completion fluid. Typical systems contain several individual grinds of particles combined in various job specific ratios. Job designs using the conventional systems require skill and expertise in order to cover a wide spectrum of permeabilities and still allow maximum clean up efficiency.
To increase performance, conventional particulate fluid loss control systems have utilized starch. Starch reduces filter cake permeability at the cost of clean up efficiency.
Bridging efficiency is the key to an effective leak off control additive system. Bridging must occur quickly with minimal particle invasion. The bridging mechanism is dependent upon the particle size distribution of the leak off control material. Such distributions are normally presented on a semi-log plot of cumulative weight of the leak off control material versus a log of the particle size. Typical prior particle size distributions of this type have an s-shaped curve in the aforementioned semi-log plot.